mine is HP7. It, unlike the other books, didn't take place at Hogwarts and SO much happened it was just soooo good there are no words to describe how incredible it was.
I love all of the books,but I'd have to say 7 was my fave. My fav Potter books are just the exact opposite of the order, #1 Deathly Hallows #2 Half-Blood Prince #3 Order of the Phoenix #4 you get the idea.
I don't like Harry Potter and the Order of the Phoenix because all it is, is Harry being miserable, Dumbledore completely ignoring Harry, Umbridge ruining the fun, and SPOILER ALERT! Highlight over there if you would like to see --> Sirius dies!